During the course of manufacture of pressurized-water nuclear reactor vessels, it is necessary to carry out certain machining or welding operations on the bottom or top ends of these vessels, consisting of spherical domes of large size and considerable mass.
For example, it is necessary to incorporate instrumentation tubes into the bottom end of the vessel and adaptors for control rod mechanisms into its top end, holes for passing through these tubes or adaptors being provided in the spherical ends of the vessel.
The incorporation of these components necessitates welding of the tube or adaptor into the spherical end at the place where it is made to pass through.
However, it is not possible to weld the tube directly onto the spherical end and it is necessary to place a relatively thick deposit of a joint metal on the portions of the spherical end surrounding the passage. Hence, facings which have certain profiles are machined, and a layer consisting of a nickel alloy of small thickness is deposited thereon.
This localized building-up of the spherical end, called buttering, can be carried out under satisfactory conditions only if the deposit is carried out in a customary welding position. This welding or building up may be effected, for example, on the flat or like a ledge.
The distribution of the facings of which it is required to provide the buttering on the inner face of the spherical dome necessitates a modification of the orientation of this dome according to the location of the facing being worked upon.
Hence it has been proposed to place the spherical dome on a handling device consisting of a table which can turn and tilt, so that its positions can be varied as the buttering work proceeds upon the several facings. For this purpose, the spherical dome is attached to the plate of the device by way of a frusto-conical skirt attached to the dome and to the plate upon which it is resting.
As the spherical ends of nuclear reactor vessels have very large masses and the tilting torques for obtaining all of the working positions are very high, it is necessary to employ handling devices or positioners of great power. It is necessary, for example, in the case of the spherical ends of nuclear reactor vessels being constructed at present, to have available positioners capable of developing a torque of 50 tone/m at a minimum.
Moreover, it is necessary in a workshop for the manufacture of nuclear reactor vessels to have available a fairly large number of positioners' in order to avoid saturation of these installations upon which the spherical ends must remain for a relatively long period in order to carry out all of the necessary welding and building-up operations.